The present invention relates generally to advanced CDMA wireless communication networks, and in particular to improved out-of-sync detection in Continuous Packet Connectivity mode.
Universal Mobile Telecommunications System (UMTS) is a third-generation (3G) wireless communication technology. The radio access network of UMTS, the UMTS Terrestrial Radio Access Network (UTRAN), provides wireless connectivity for both circuit-switched and packet-switched traffic between User Equipment (UE) and the core network, via Radio Network Controllers (RNC). Each RNC controls one or more Base Stations (BS), which provide the air interface connectivity to mobile UEs. Wideband Code Division Multiple Access (WCDMA) is one technology employed in UTRAN implementation.
High-Speed Downlink Packet Access (HSDPA) is a mobile telephony communications protocol for delivering packet data at high transfer speed and capacity. Downlink channels (i.e., transferring data from BS to UE) in the HSPDA protocol lack the variable spreading factor and fast power control of traditional CDMA, and instead use adaptive modulation and coding, fast packet scheduling at the BS, and transmit at full residual power. Hybrid Automatic Repeat-Request (HARQ) is employed for fast retransmission of corrupted packets.
High-Speed Uplink Packet Access (HSUPA), defined in UMTS Release 6, is a corresponding protocol for high-speed packet transfer in the uplink direction (i.e., from UE to BS). HSUPA uses an Enhanced Dedicated Channel (E-DCH) that features a short Transmission Time Interval (TTI) for fast link adaptation and HARQ for error protection. The HSUPA packet scheduler operates on a request-grant basis, wherein UEs request permission to send data and a scheduler at the BS schedules uplink transmissions from among multiple UEs.
In Release 6 E-DCH, uplink control information is sent on a continuing basis by the UE to the BS on the E-DCH Dedicated Physical Control Channel (E-DPCCH). The UE transmits data to the BS as scheduled on the E-DCH Dedicated Physical Data Channel (E-DPDCH). In HSDPA, downlink control information is sent by the BS to the UE on the High-Speed Shared Control Channel (HS-SCCH), and downlink data packets are transmitted on the High-Speed Downlink Shared Channel (HS-DSCH).
Release 7 of the UTRAN specification defines “continuous connectivity for packet data users,” or simply, Continuous Packet Connectivity (CPC). CPC enhances system capacity to support a very large number of packet-oriented users by reducing signaling overhead, uplink interference, and downlink transmission power. Unlike non-CPC UTRAN, CPC mode employs both uplink and downlink closed loop power control. In the uplink, Transmit Power Control (TPC) commands are sent from UE to BS, along with pilot symbols, on the E-DPCCH. In the downlink, the Fractional Dedicated Physical Channel (F-DPCH) carries only TPC commands to each individual UE. UEs use the TPC commands for both DL inner loop and outer loop power control, and make channel estimates based, at least in part, on the received F-DPCH signal. The UE uses the channel estimates, in turn, to generate TPC commands for the BS, transmitted in UL DPCCH signals.
During normal operation, as received signal quality degrades, the UE sends “up” TPC commands to the BS to increase the transmit power of dedicated channels. When the signal quality drops below a predetermined threshold for a predetermined time, the UE notifies the network that it is “out-of-sync,” and terminates transmissions. This is to preclude generating excessive interference in the downlink by perpetually increasing the BS transmit power with “up” TPC commands. Higher-order network layers handle an out-of-sync UE. Since going out-of-sync disrupts communications, the UE bases an out-of-sync determination on averaged channel quality measurements taken over time, to avoid overreacting to short-term channel fluctuations. For example, in many implementations, the TPC command error rate should exceed 30% over a 160 ms measurement interval to trigger an out-of-sync determination.
One feature of CPC mode is discontinuous (DTX) uplink (UL) DPCCH transmission when the UE is not transmitting data, also commonly known as UL DPCCH gating. The network defines an UL DTX active period, typically on the order of a few timeslots, during which the UE transmits UL DPCCH signals and receives F-DPCH signals containing valid TPC commands. During an UL DTX gated period, which is typically a few tens of ms, the UE suppresses UL DPCCH transmissions to reduce uplink interference and preserve battery power. To further preserve battery power, the UE ceases monitoring F-DPCH signals during the UL DTX gated period. The transmission of the UL DPCCH power control preamble shall start NPCP PCP radio frames prior to the radio frame where the uplink DPDCH/E-DPCCH/E-DPDCH transmission starts, where NPCP PCP is a higher layer parameter set by UTRAN; During the uplink DPCCH power control preamble, independently of the selected TFC, no transmission is done on the DPDCH/E-DPCCH/E-DPDCH.
One problem with the discontinuous UL DPCCH transmission of CPC mode is that if the UL DTX gated period is sufficiently long, the UE has fewer opportunities to assess channel quality, and accordingly may have insufficient data to make reliable out-of-sync decisions. The current specification requires UE to monitor the last received 240 TPC commands for the detection of out of sync. Thus if UL DTX gated period is set to several tens of ms the out of sync detection will also become proportionally longer. The UE in out-of-sync will attempt to return to in-sync state and report the status to the network. For in-sync detection the UE is also required to monitor the same number of last received TPC commands (i.e. 240). Thus according to the current specification if UL DTX gating period is long then the in-sync detection will also become considerably longer.
Furthermore, because there is no specified timing requirement at the BS between reception of an UL DPCCH signal and transmission of a F-DPCH signal, the UE cannot ascertain with certainty that a received F-DPCH contains TPC commands derived from the last UL DPCCH signal the UE transmitted, at least at the beginning of each UL DTX active period. For example, a BS may transmit F-DPCH signals periodically or continuously during the UL DTX gated period. Since the UE must ignore these transmissions, there is no requirement that they contain “valid” TPC commands—that is, TPC commands calculated in response to the last dedicated channel signal sent by the UE—or that the F-DPCH signal power level be responsive to the last TPC commands sent by the UE. Accordingly, at least at the beginning of an UL DTX active period, there is a non-trivial probability that at least the first F-DPCH signal the UE receives may be invalid. Reliance on invalid F-DPCH signals further degrades the integrity of out-of-sync determination.